Pressure measurement in vacuum systems is particularly challenging because of the enormous range of pressures that can be realized. Typical vacuum systems have two or more types of gauges, each with its particular range of usefulness. The need to switch between different vacuum gauges is tiresome and produces reading discontinuities where the gauge ranges meet. Gauges with broad pressure ranges are attractive because they can reduce the number of different types of gauges needed to monitor a particular vacuum system.
One class of pressure gauges, known as molecular drag gauges, makes use of the phenomenon that at low pressures the drag forces produced by a gas on an object moving through it are proportional to the pressure of the gas.
One type of gauge employing this principle uses a freely swinging fiber or vane as a means of measuring pressures in the range of 10.sup.-3 Torr to 10.sup.-5 Torr. The fiber or vane pendulum is started swinging mechanically, for example by mechanically shaking the vacuum container. The damping is due mainly to the gas in the container. The time for the pendulum to damp to one half its original amplitude, or half-life, is measured. The relationships between damping and pressure and damping and half-life can be used to determine the pressure as a function of half-life. This method of measuring pressure, however, is quite limited in range. It is also cumbersome and takes on the order of one hour to make a measurement at low pressure.
Another type of gauge employing this principle uses a tuning fork made from piezoelectric material as the sensing element. The tuning fork is made to oscillate and its resonance resistance is directly proportional to gas pressure when the pressure is low enough to be in the molecular flow region. When the pressure rises to a level where the flow begins to become viscous, the resonance resistance continues to increase with pressure, but at a much reduced rate. To make a pressure measurement using the tuning fork oscillator, the tuning fork is placed where the pressure is to be measured and caused to oscillate by means of an oscillator circuit. The pressure is determined by measuring the difference between the resonant resistance where the pressure is being measured and the natural resonance resistance of the tuning fork. One of the drawbacks of this device is that its range is limited at the low end when the resistance caused by the gas is of the same order as the natural resonance resistance of the tuning fork. The sensitivity is also limited at the high end by the shift from molecular resistance to the transition between molecular and viscous resistance.
Yet another type of pressure gauge that makes use of the drag forces of a gas is called a spinning rotor gauge. This gauge measures the deceleration of a magnetically levitated spinning metal sphere inside a stainless steel chamber that is, in turn, immersed in the gas that is to have its pressure measured. The ball is electromagnetically spun up to a target rotation rate and then allowed to decelerate. The rate of the ball's deceleration is proportional to the number of gas molecules that come in contact with the ball per unit time which is, in turn, proportional to gas pressure. This gauge can measure pressures in the range of 10.sup.-2 Torr to 5.times.10.sup.-7 Torr. Spinning ball gauges are very accurate, however their use, is restricted by their size, high cost and limited range of measurement capability.